$ B = \left[\begin{array}{rr}3 & 0 \\ -2 & -2\end{array}\right]$ $ D = \left[\begin{array}{rrr}1 & 3 & 1 \\ 1 & -2 & 4\end{array}\right]$ What is $ B D$ ?
Solution: Because $ B$ has dimensions $(2\times2)$ and $ D$ has dimensions $(2\times3)$ , the answer matrix will have dimensions $(2\times3)$ $ B D = \left[\begin{array}{rr}{3} & {0} \\ {-2} & {-2}\end{array}\right] \left[\begin{array}{rrr}{1} & \color{#DF0030}{3} & \color{#9D38BD}{1} \\ {1} & \color{#DF0030}{-2} & \color{#9D38BD}{4}\end{array}\right] = \left[\begin{array}{rrr}? & ? & ? \\ ? & ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ B$ , with the corresponding elements in column $j$ of the second matrix, $ D$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ B$ with the first element in ${\text{column }1}$ of $ D$ , then multiply the second element in ${\text{row }1}$ of $ B$ with the second element in ${\text{column }1}$ of $ D$ , and so on. Add the products together. $ \left[\begin{array}{rrr}{3}\cdot{1}+{0}\cdot{1} & ? & ? \\ ? & ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ B$ with the corresponding elements in ${\text{column }1}$ of $ D$ and add the products together. $ \left[\begin{array}{rrr}{3}\cdot{1}+{0}\cdot{1} & ? & ? \\ {-2}\cdot{1}+{-2}\cdot{1} & ? & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ B$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ D$ and add the products together. $ \left[\begin{array}{rrr}{3}\cdot{1}+{0}\cdot{1} & {3}\cdot\color{#DF0030}{3}+{0}\cdot\color{#DF0030}{-2} & ? \\ {-2}\cdot{1}+{-2}\cdot{1} & ? & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rrr}{3}\cdot{1}+{0}\cdot{1} & {3}\cdot\color{#DF0030}{3}+{0}\cdot\color{#DF0030}{-2} & {3}\cdot\color{#9D38BD}{1}+{0}\cdot\color{#9D38BD}{4} \\ {-2}\cdot{1}+{-2}\cdot{1} & {-2}\cdot\color{#DF0030}{3}+{-2}\cdot\color{#DF0030}{-2} & {-2}\cdot\color{#9D38BD}{1}+{-2}\cdot\color{#9D38BD}{4}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rrr}3 & 9 & 3 \\ -4 & -2 & -10\end{array}\right] $